Riser tensioning system

ABSTRACT

In a substructure ( 10 ) for a floating oil or gas production platform, an arrangement to tension a plurality of risers ( 16 ) extending from the sea bed up to the substructure, the arrangement comprising: a conventional hydraulic tensioner/heave compensator ( 17 ) for each riser, in which there is a soft spring formed by a piston cylinder combination acting against an accumulator, the heave compensators for the risers being disposed to compensate for vertical oscillations of relatively short period (e.g. from 1 second to about 5 minutes) between the risers and a vertically adjustable Xmas tree deck ( 18 ); and a vertical position adjustment system ( 21/22 ) capable of intermittent operation to adjust the vertical position of the Xmas tree deck ( 18 ) relative to the floating substructure ( 10 ) to compensate for longer term changes which would otherwise cause the individual riser&#39;s tension or stroke position to depart from its target value/range; the Xmas tree deck vertical position adjustment system being normally located in one particular position within its range of movement to compensate for the longer term changes.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a riser tensioning system for a floating oil orgas production platform.

In particular, the invention relates to a riser tensioning system foruse on a deep draft floating production facility of the type illustratedin PCT Application WO99/10230. However, the invention could also be usedon other floating platforms.

BACKGROUND OF THE INVENTION

Oil and gas production is taking place in progressively deeper water. Inwater depths up to about 300 m in the North Sea and about 400 m in theGulf of Mexico, fixed platforms have been used. In deeper water depths,floating platforms are necessary. Production has taken place from shipshaped vessels, column stabilised semi-submersible vessels, floatingspars and tension leg platforms (TLPs).

In all cases, near vertical pipelines bring the oil (or gas) up from thesea bed to the floating platform for processing and onward transmission.These near vertical pipelines are known in the offshore industry as‘risers’. A problem exists in that risers need to be held constantly intension against vertical motions (‘heave’) of a floating platform. Ifthe risers are allowed to go into compression, buckling may occur. Thusit has been necessary to use heave compensators to keep the risers undertension.

In water depths greater than 1500 m, the heave period becomes a problemfor TLPs. Deep Draft Floaters (DDFs) have smaller motions thanconventional semi-submersible vessels, but larger motions than TLPs.

In some floating platforms, such as in ‘spar’ platforms, it has beenknown to use external buoyancy cans to tension the risers. Thistechnique is described in U.S. Pat. No. 4,702,321. Tensioning withexternal cans has several drawbacks. The risers are confined in acentral vertical duct. Damage from fatigue may be experienced by therisers due to uncontrolled ‘piston’ actions from buoyancy cans andexcitation of various modes of vibrations, as well as uncontrolledsticktion phenomena. This may lead to rupture and consequential leakage,fire and explosion with resulting damage to the topside facilities andto other risers. This makes caisson type vessels especially vulnerable.In these vessels, the leakages pass up through the caisson well into themiddle of the topside deck installation. TLPs do not have thisdisadvantage as their risers are suspended freely in the water. In mostcases a leakage in the riser system will be dispersed from the TLP bywater currents and winds at the surface.

In principle, it is possible to extend (lengthen) tensioner systemsdeveloped for TLPs to accommodate the larger heave motions which arelikely to be experienced by risers on DDFs and other vessels. However,this creates practical difficulties.

DDFs have slightly less air gap than TLPs between their lowest deck andthe sea surface, because there is no “pull-down” from the tethers as theTLP moves off its nominal position. The same effect increases the needfor riser “pay-out” for a DDF for the same displacement Additionally,for DDFs, there is a contribution from their significantly larger heavemotions. To allow for this larger pay-out/pay-in of risers, (oftenreferred to as heave compensation), the traditional ‘pulling cylinder’design of heave compensator would become so long that under normaloperation, the ‘tensioner ring’ would be partly below water level. Thetensioner ring is an assembly connecting the tensioner rods of the heavecompensator to the riser. If the tensioner ring is partly below water,this critical connection is difficult to reach for inspection.

To raise this critical connection to above sea level, it would benecessary for the tensioner rods to be longer, so that they would extendup through the deck opening. This would lead to a complex arrangement,with a risk of potential clashes, or loss of valuable area on theproduction deck or drilling equipment deck. Another expedient forraising the tensioning ring would be to invert the tensioner system, sothat it had a ‘rods up’ configuration. This would increase the Xmas treeheight above the tree deck; lead to instability in shear and torsion;and possibly lead to a compression/buckling problem with the invertedtensioner rods.

In any case, the tensioner stroke necessary for such longer tensionersystems could be beyond what is practical, reliable and cost effective.

Thus there is a requirement for a riser tensioning system which would beapplicable to vessels with larger heave motions than TLPs, and whichwould avoid the practical difficulties outlined above.

DISCLOSURE OF THE INVENTION

The theoretical background to this invention is described in OTC Paper11904 (published at Houston Tex. in May 2000).

The invention provides, in a substructure for a floating oil or gasproduction platform, an arrangement to tension a plurality of risersextending from the sea bed up to the substructure, the arrangementcomprising:

i) a conventional hydraulic tensioner/heave compensator for each riser,in which there is a soft spring formed by a piston cylinder combinationacting against an accumulator, the heave compensators for the risersbeing disposed to compensate for vertical oscillations of relativelyshort period (e.g. from 1 second to about 5 minutes) between the risersand a vertically adjustable Xmas tree deck, and

ii) a vertical position adjustment system capable of intermittentoperation to adjust the vertical position of the Xmas tree deck relativeto the floating substructure to compensate for longer term changes whichwould otherwise cause the individual riser's tension or stroke positionto depart from its target value/range; the Xmas tree deck verticalposition adjustment system being normally located in one particularposition within its range of movement to compensate for the longer termchanges.

In the foregoing, examples of the relatively short period oscillationsreferred to in i) are the first order wave motions and normaloperational state surge, sway and pitch slow drift oscillations.Examples of the longer-term changes referred to in ii) are an extremequasistatic horizontal offset caused by severe storm conditions, extremeoverlaid oscillations at the critical surge/sway period of the mooredsubstructure (slow drift), or inadvertent flooding of one of the buoyantcompartments of the substructure.

It is preferred that the vertical position adjustment system includesstiff hydraulics (in which pistons may be hydraulically locked) whichInterconnect the Xmas tree deck and the substructure.

It is further preferred that hydraulic oil is supplied from pressurizedaccumulators when raising the Xmas tree deck, and bled to a tank whenlowering the Xmas tree deck.

In one preferred form the Xmas tree deck has counterbalance means, suchthat its vertical movements to compensate for longer term changes arecounterbalanced, and only minimal force is required to effect verticalmovement.

In this form it is preferred that the Xmas tree deck vertical positionadjustment system comprises at least three piston cylinder andaccumulator combinations acting between the Xmas tree deck and thefloating substructure, and in which the three combinations aresynchronised to avoid excessive tilt of the Xmas tree deck relative tothe substructure.

It is further preferred that the cylinders in the combinations areconnected to a single accumulator, so that the Xmas tree deck issensibly horizontal, and in which there is a rack and pinion mechanismwhich engages with the substructure to maintain parallellity of themoving X-mas tree deck with the substructure at all times, where rackand pinions engage at least two faces of the deck, at right angles.

In this form it is alternatively preferred that the vertical positionadjustment system comprises at least three pulley systems acting betweenthe Xmas tree deck and the substructure, and in which the pulley systemsare powered to compensate for longer term vertical changes.

It is further preferred that a part of each pulley system is engaged bya further piston cylinder combination.

The pulley systems may be powered by hydraulic or electric motors forsynchronous movement.

In forms of the invention wherein the Xmas tree deck has counterbalancemeans, it is preferred that there is means whereby synchronism can beeffected by hydraulic valve logic while the vertical position adjustmentsystem is moving.

It is further preferred that a locking provision on the verticalposition adjustment system is arranged to become unlocked when a heavecompensator is approaching the end of its stroke.

It is still further preferred that predetermined high and low pressuresin the heave compensators are arranged to open valves between the pistoncylinder combinations and the accumulators in the vertical positionadjustment system.

In another form of the invention, it is preferred that the verticalposition adjustment system includes mechanical engagement devices whichinterconnect the Xmas tree deck and the substructure.

In any of the forms of the invention described above, there may be acontrol system which has provision for the arrangement to operatewithout human intervention (e.g. in circumstances in which the floatingoil or gas platform is temporarily de-manned during a hurricane).

It is preferred that the control system includes a program to adjust theelevation of the Xmas tree deck in response to stroke measuring deviceson at least three of the individual heave compensators, whereby, atpreset limits of compensator stroke, the vertical position adjustmentsystem moves the Xmas tree deck in a sense towards the limit reached onthe individual heave compensator.

It is further preferred that the spring rates of the individual heavecompensator are increased near both the limits of travel of theindividual heave compensators, such that the equilibrium of the balancedXmas tree deck will be changed so that the Xmas tree deck moves towardsthe applicable limit of travel under the action of the vertical positionadjustment system.

In any of the forms of the invention described above, the balancedvertical position adjustment system under mean force equilibrium may benormally retained by frictional forces in one particular position withinits range of movement, and is moved intermittently in direct response toone or several of the heave compensators approaching a limit ofoperation.

It is preferred that hydraulic cylinders in the vertical positionadjustment system are pre-pressurised, so that the system acts as aprecompressed spring which fails to ‘safe’ if the active drive systemslose pressure.

It is further preferred that the heave compensators have an increasedvertical spring stiffness as they approach the ends of their strokeranges.

Preferably, there is adjustment means to change the characteristics ofindividual heave compensators, so that both the heave compensators forthe risers and the vertical position adjustment system for the Xmas treedeck approach their limits of operation at the same time.

In any of the forms of the invention described above the Xmas tree deckmay have an integral deck centralisation system.

It is preferred that the Xmas tree deck is supported on at least fourpairs of vertical position adjustment systems disposed generallysymmetrically about the deck, whereby to centralise the deck within agenerally horizontal aperture in the substructure, such that individualheave compensators react lateral loads from individual risers into theXmas tree deck, and the Xmas tree deck as a whole is centralised withinthe horizontal aperture.

It is further preferred that vertical rods guide the Xmas tree deckwithin the horizontal aperture, or that projections from the Xmas treedeck engage vertical guide rails surrounding the horizontal aperture, orthat there are pinions on the Xmas tree deck arranged to engage verticalracks round the horizontal aperture.

In a form in which there are pinions, it is further preferred thatresilient means are disposed to hold the pinions in engagement with theracks.

In any one of the forms of the invention described above the verticalposition adjustment system for the Xmas tree deck may have a generallycentral slot occupied by dillstring riser tensioner, so to facilitatedrilling/workover.

The Xmas tree deck may be used as a foundation for a drilling risertensioner, or for a workover drillstring tensioner.

Conveniently, hoses from individual Xmas trees on the Xmas tree deck areled through individual downwardly opening trumpet sleeves dependant froma platform above the Xmas tree deck.

Optionally, there are several individual bays of deck grid systemsplaced onboard the substructure to reduce the pitch differential acrossthe riser array.

Advantageously, there is provision to lock off the vertical positionadjustment system for the Xmas tree deck, whereby to adjust the verticalheave stiffness of the substructure.

The invention includes a substructure for an oil or gas productionplatform, and having an arrangement as described above.

The invention also includes a method of controlling the tension inrisers extending from the sea bed up to the hull of a substructure for afloating oil or gas production platform, using the arrangement asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Some specific embodiments of the invention (and some variants thereof)will now be described by way of example with reference to theaccompanying drawings, in which:

FIGS. 1, 1 a and 2 are diagrammatic side elevational views of asubstructure for a floating oil or gas production platform, showingriser tensioning systems, where FIG. 1a illustrates the effect of pitchor heel;

FIGS. 3 and 4 are diagrammatic side elevational views (to an enlargedscale) of Xmas tree decks incorporated in the substructures of FIG. 1 orFIG. 2;

FIGS. 5, 5 a and 5 b are a plan views of an Xmas tree deck and details;

FIG. 6 is a disturbance/time graph illustrating operation of theinvention;

FIG. 7 is an isometric illustration of a vertical position adjustmentsystem forming part of the invention;

FIG. 8 is a plan view of another embodiment of the invention;

FIG. 9 is a view on arrow IX in FIG. 8;

FIG. 10 is an isometric view of the same embodiment;

FIG. 11 is another isometric view of that embodiment;

FIG. 12 is a side view showing the use of a workover rig;

FIG. 13 is a diagrammatic view showing operation of the system; and

FIGS. 14 and 15 are disturbance/time graphs illustrating operation ofthis embodiment.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 shows a substructure 10 for a floating oil or gas productionplatform. The substructure has a set of legs 11 (only two of which areshown) upstanding from pontoons 12. The substructure 10 floats with itspontoons 12 below the sea level 14. The substructure has a main deck 15for facilities or ‘topsides’ (not shown).

The substructure is subject to various ‘ship motions’ (such as heave,surge, sway, pitch, roll, and yaw), and slower oscillations due toreactions of the mooring system—these can be referred to as cyclicmotions.

Other motions can be categorised as slowly acting or discrete motions.These include longer term displacements caused by storm surges, offsetscaused by currents and the long term effects of wind and wave, andchanges in draft caused by e.g. damage to a compartment leading toflooding, or to mooring line failure. These motions can produce bothriser stroke variations of relatively short period, and also longer-termchanges like a change in draft or sea level changes relative to the seabed.

For a DDF (as illustrated in WO99/10230) the cyclic motions may create aneed for riser stroke compensation in the range of 2.5 m to 3.5 mdepending on local environmental conditions. These oscillations can takeplace continuously over long periods and will include slow driftcomponents under normal operating conditions. Longer-term variations mayunder extreme conditions necessitate stroke adjustment in the order of 6m to 8 m, but will take place at rare intervals e.g. twice a during adesign storm.

Oil or gas from subsea wells is brought up to the platform by risers 16.The lower ends of the risers (not shown) are fixed to wells on the seabed. The upper ends of the risers are supported in heave compensators17, which are mounted below an Xmas tree deck 18. The risers 16 lead toXmas trees 19 standing on the deck 18.

Following the invention, the position of the Xmas tree deck 18 isarranged to be moveable vertically with respect to the substructure.Vertical movement is represented in FIG. 1 as being effected by directmechanical connections 21 to prime movers 22. This movement couldalternatively be effected by mechanisms generally similar to those usedto operate the legs of jack-up platforms. A substructure mounted guidingsystem 21 a is shown for the Xmas tree deck 18.

The principle of operation is that short period oscillations are ideallycompensated by the heave compensators 17, while the effects of longerterm changes can be counteracted by vertical movement of the Xmas treedeck 18.

FIG. 1a shows the working principles of the arrangement when thesubstructure is subject to angular displacements (pitch/roll/heave). Therisers 16 will remain near vertical and the heave compensators 17 willhave to produce differential strokes across the Xmas tree deck 18 inorder to maintain a constant tension. The deck 18 will always remainparallel to its environment Differential strokes may also be caused bythe well layout pattern at the sea bed being different from the wellpattern on the Xmas tree deck.

It will be understood that there are connections 21 at each corner ofthe Xmas tree deck 18; and that the operation of these connections issynchronous to avoid tilt of the deck 18. The connections 21 can belocked in one particular vertical position within their overall range ofvertical operation.

FIG. 2 shows a variant of FIG. 1, in which the Xmas tree deck 18 issuspended in a counter balanced mode by diagrammatic springs 23. Thishas the effect of reducing the power required in the prime movers 22 tomove the Xmas tree deck vertically. Heave compensators 17 ideallycompensate for short-term oscillations as before, as well as for angulardisplacements (pitch/roll/heel/list)

FIG. 3 shows in more detail the arrangement of the heave compensators 17and the vertical position adjustment systems, as shown in FIGS. 1 and 2.In this case cylinders 24 are connected to pneumatic accumulators (gasover oil—not shown) to balance the system for easy vertical movement.(In another form, control of the balanced system could be effected by arack and pinion drive.) In FIG. 4 the stroke required of the cylinders24A is reduced by using a pulley system 25.

With the arrangements shown in FIGS. 3 and 4, the cylinders 24/24A maybe shut off to lock the Xmas tree deck 18 in a particular verticalposition. Alternatively, the locking may be effected mechanically.Synchronism of the cylinders at the four comers can be effected byhydraulic valve logic when the system is in operation.

In accordance with a feature of the invention, the cylinders 24/24A canbe brought into operation automabcally when the heave compensators 17are near the end of their stroke in one sense or the other. Automaticoperation may be appropriate if the platform has to be temporarilyabandoned while in the path of a storm. Pressure controls in the heavecompensators 17 can be arranged to open up valves automatically, tooperate the cylinders 24/24A and their associated accumulators.

FIG. 5 shows details in plan of the arrangements illustrated in FIGS. 3and 4. The Xmas trees are disposed in a three by three array. Cylinders24 are located at the corners of the Xmas tree deck 18. Structuralguides 26 (detailed in FIG. 5A) constrain the deck 18 for verticalmovement relative to the substructure. Each well has a TLP type heavecompensator 27 with four cylinders 28. A manifold 29 is shown(diagrammatically) receiving the production from the wells via flexiblehoses 29A (in a manner known from TLPs). The load on the deck 18 couldconsist of tension on nine risers (at 0.98 MN each) amounting to 9.81MN, the weight of the heave compensators totaling 2.4 MN, and the weightof the deck 18 at 3.45 MN, adding up to 13.35 MN.

A simplified time history of operation of the combined arrangement isshown graphically in FIG. 6. Responses are based on detected heave onlybut may be driven partly by coupled pitch effects. The trajectory of atypical riser measured at the tensioner ring is designated 31; and theintermittent movement of the vertical position adjustment system isdesignated 32. Initially the heave compensator operates between upperand lower thresholds, designated 33 and 34. When, due to a combinationof oscillations and longer term changes, the down stroke of one heavecompensator passes the lower threshold at 35, the vertical positionadjustment system is brought into operation. This moves by apredetermined incremental value from 36 to 37, setting up new upper andlower thresholds 38 and 39. Note that the thresholds 33/34 and 38/39will fluctuate as a function of the pitch/heel angle. Large pitch angleswill lessen the effective heave compensator stroke available beforecrossing set thresholds, as parts of the stroke will be reserved forcompensation of the pitch/heel action. The objective of the switchinglogic would be for the heave compensators 17 always to be operating intheir ‘mid stroke’ position. Each time the possibility of ‘bottomingout’ is detected in one of the comer compensators (e.g. 17A in FIG. 5),the vertical position adjustment system would effect a shift of say 1.5m.

The vertical positioning system may contribute to increased overallsystem redundancy. The vertical position adjustment system may bedesigned to operate whenever there is a calculated possibility of one ofthe heave compensators 17 over stroking. Secondly, the vertical positionadjustment system would operate should one heave compensator 17fail/loose tension. In the case of a typical low probabilityfailure—like a double failure induced loss of hydraulic power (bursthoses) on a heave compensator—the Xmas tree deck 18 can be designed tomove vertically until a minimum riser tension is restored for the failedheave compensator. The implication would be a slightly increased tensionon the remaining intact heave compensators and narrowing in effectivethresholds for engaging the vertical positioning adjustment system,since the heave compensators 17 would have less net stroke capacityavailable in this mode. Thirdly, if there were any possibility ofcompression occurring in a riser 16, the Xmas tree deck 18 would beraised until tension was ensured. The above scenario accounts for rareevents (at accidental probabilities) and may not account for alleventualities. Possible impact loads from failing heave compensators(overstroking or bottoming out) will normally happen close to theextreme of a displacement of the stroke, which means that the velocitieswill be low, giving relatively minor impact loads, or sufficient timefor the system to respond to avoid impact.

A diagrammatic view of the Xmas tree deck 18 is presented in FIG. 7.This illustrates a possible mechanical deck adjustment control system.The Xmas tree deck 18 can be arranged with an active drive and/orsynchronization system to control the Xmas tree deck motion andparallelity with its environment at all times. A rack and pinion systemshown as 41/42 could be designed as a passive slave system (with thedeck riser load carried by another system not shown); or it could be apowered system driven by hydraulic cylinder 24 or direct actingelectrical/hydraulic powered motors (not shown). Rack and pinion gears41/42 will provide the deck parallel synchronization (with two racks atright angles as a minimum) and will at the same time give verticalguidance.

Basic operation of the vertical position adjustment system would bepossible with stiff independent hydraulics. These can operate in asynchronous manner given sufficient hydraulic power. To save on powerneeded to drive the hydraulics, pressurised accumulators could beconnected with the cylinders to lift the deck 18 during circumstancesleading to power peaks. Downward motion would be effected by bleedinghydraulic fluid to a buffer tank.

A further advantage would be gained by arranging a counterbalanced or‘weightless’ deck. In this case the hydraulics would be fully balancedon cylinders with pressure supplied from accumulators, and could be setmanually each time the load on the deck 18 changed. Load cells could beused to detect individual loads. A rack and pinion drive could be usedin this case to control deck guiding and parallellity with itsenvironment at all times, but also for active driving of the balanceddeck. The system could be configured so that the cylinders remain fixeduntil a stroke measuring and surveillance system on the heavecompensators invokes en active repositioning of the active deck drivemechanism. Alternatively, the balanced deck can be designed such that aglobal tension drop/increase across the entire deck 18 will build up thenecessary delta force to get the deck to move from the close to constantpressure of the accumulators. In this case the heave compensators wouldbeneficially be designed to work on a non-linear (hardening spring)characteristic. This might be advantageous in a non-manned (hurricane)condition, as the entire system will be passive, with no active controlsystem.

Another example of the invention will now be described, with referenceto FIGS. 8 to 15.

In this example, a ten well system has been considered for the selectionof equipment in a 915 m water depth. This system has ten risers 116arrayed around a three by four matrix, with each riser having a 1.34 MNnull tension. This example has been developed extensively, and will beexplained in detail. The same theory applies for deeper water depths.The requirements and equipment selection for 2133 m of water (withnominal riser tensions of 4.45 MN) are practical with current hardware.

FIGS. 8 and 9 show two basic spring components for a two-tier risertensioner system. These components comprise traditional heavecompensators 117 (HF); and ram style Xmas tree deck support cylinders124 (LF). (The cylinders 124 form a vertical position adjustmentsystem.) These two spring components coupled in series address the risermotion requirement for a floating substructure, in this case a DDF. Theother major component of the system is the Xmas tree deck 118 andintegral centralizing system. Xmas trees 119 are located on the deck118. Flexible hoses 129A are connected to the Xmas trees.

Some of the parts illustrated in FIGS. 9 to 12 are proprietary itemsdesigned by Hydralift Inc. No claim is made to these individual items inthe present application.

The HF system operates continuously to accommodate the first order risermotions. This is a conventional heave compensator system with 3.81 m ofstroke. This stroke length was selected to account for 1.52 m ofvertical riser upstroke, 2.13 m of vertical riser downstroke and maximumanticipated 2 degrees of angular riser displacement (inside the Xmastree deck). Ten heave compensators 117 are arranged around the outsideslots of the three by four well bay matrix. The two inside slots(109,110) are reserved for well workover or ROV operations. As shown inFIG. 9, each heave compensator 117 has four tensioner elements, eachcomprising a cylinder 108, an accumulator bottle 107, and a risercentralizer arm 106 attached to the Xmas tree deck 118.

Each tensioner element 106-108 is independent and consists of a cylinderconnected directly to a dedicated gas expansion accumulator. Thecylinder blind end is suspended from a single point on the deckstructure with the cylinder rods connected directly to the productionriser spool joint. A total of 1.34 MN is to be transferred from therisers 116 into the Xmas tree deck 118 at each well bay slot. Gasexpansion accumulators are placed on the deck 118. They are positionedon one side of the well bay to allow the well tree's flow lines to loopthrough the deck to a common pipe header fixed to the DDF. Four risercentralizers 106 are integrated into each well bay slot to fix the riserin the center of the slot.

The accumulators, charged to a pressure of 12.2 MPa, act within theannulus of a cylinder with a 216 mm I.D. bore and a 102 mm O.D. rod, toproduce the required null riser tension of 1.34 MN. The 0.72 m3 capacityaccumulators result in a heave compensator stiffness of 72.0 kN/m fromthe null position to the full downstroke position. Therefore, all tenheave compensators combine to generate a total HF system stiffness of720 kN/m when stroking out.

During emergencies, the heave compensators 117 are designed to operateat the required tension ranges using only three of their four tensionerelements 106-108. This approach insures that when three cylinders 108are operating, full tension can be supported without exceeding themaximum rated design pressure of 21 MPa at the full downstroke position.The cylinder bore size is selected to optimize the pressure rating ofthe cylinder per unit weight of the tensioning element.

The LF System operates only when necessary, in response to large lowfrequency displacements or discrete events. A total stroke range of 8.84m is chosen to accommodate all LF motion requirements. This overallvertical stroke length is broken down to 3.96 m of upstroke and 4.88 mof downstroke. As shown in FIG. 11, traveling guides 143 constrain theXmas tree deck 118 on fixed vertical guides 144.

To support the Xmas tree deck 118, twelve ram type cylinders 124 arearranged in six pairs. Three pairs are located along one side of thematrix that has four well slots. Three more pairs are located directlyacross the matrix, to produce a balanced support of the Xmas tree deck.Each cylinder 124 is independent and is connected directly to adedicated gas expansion accumulator. A collar is designed into the rodend of each cylinder and is supported by structure tied back into themain well bay of the DDF. The cylinders 124 operate in a rod upconfiguration with a chain sprocket 125 attached to the end of the rod.A support chain 126 is terminated at the cylinder support structure,runs up and over the chain sprocket, then down to terminate at the Xmastree deck 118. This arrangement could be considered as similar to adrilling riser tensioner with only two parts of wire or chain. Thisconfiguration results in a compact arrangement to maximize the verticaltravel of the Xmas tree deck.

Standard ram cylinders 124 with 4.42 m of stroke generate the required8.84 m of platform travel. The gas expansion accumulators are mounted inpairs along the inside walls of the main well bay. Rollers or bearingsare mounted in each corner of the Xmas tree deck 118 to react againstthe centralizing support structure in the main well bay of the DDF. Alllateral loads generated by the risers 116 are reacted individually bythe heave compensator centralizers into the deck 118; then, as a whole,the deck 113 is centralized within the main well bay of the DDF.

The cylinders 124 are designed to support the following summation ofloads: the deck structure, the weight of ten heave compensator systemswith fluid, the maximum riser loads that are generated by ten risers atthe full heave compensator downstroke position and half the weight often full production flow line hoses. To accomplish this, theaccumulators are charged to a pressure of 14.6 MPa acting within eachcylinder with a 470 mm I.D. bore and produces a total null support chaintension of 15.1 MN. The 1.5 m3 accumulator capacity results in an Xmastree deck LF downstroking stiffness of 870 kn/m.

During emergencies, the deck support system is designed to operate atthe required tension ranges using only eleven of its twelve supportcylinders 124. With pressures adjusted, this approach ensures that whenonly eleven cylinders are operating, full system tension can besupported without exceeding the maximum rated design pressure of 21.42MPa at the full downstroke position. The cylinder bore size is selectedto optimize the pressure rating of the cylinder per unit weight of thetensioner element. The deck structure and cylinder attachment locationsare designed to minimize deflections allowing smooth vertical motion ofthe deck even with one cylinder out of service.

The Xmas tree deck 118 can be designed to lock off at specificelevations during normal operation, thereby becoming a very stable workplatform for installation or maintenance work (see FIG. 12). A workoverrig 145 can operate through a BOP 146, and the Xmas tree deck acts as asupport for a drilling riser tensioner 147. The vacant centrally locatedslots 109 or 110 can be used for workover, and the substructure can bemoved so that the respective slot is directly over the sub-sea well.

The HF and LF systems operate in combination to cover all ranges ofplatform and riser induced motions. With the LF system initialized inthe heave compensator 117 and Xmas tree deck 118 null positions, thetotal amount of riser vertical downstroke is 7.01 m, and 5.48 m isavailable for riser upstroke. Therefore, the total range of usable risermotions, relative to the DDF, for the deck system is 12.49 m.

The combined riser tensioning system operates in a completely passivemode if a platform is abandoned during extreme environmental conditions.Hydraulic control as an override is desired for specific operations,such as platform maintenance, wellhead tree installations and extremetide adjustments. The preferred passive operation of tensioning systemis accomplished by tuning the HF and the LF systems such that they workclose to identical spring characteristics. The heave compensators 117combined will then have the same stiffness as the Xmas tree deck supportcylinders 124. Since the HF heave compensators 117 each consist of anarray of four individual cylinders 108, the motion compensation will inmost cases be picked up by these heave compensators in isolation withoutrequiring the Xmas tree deck 118 to move. This is because, for a floaterlike a DDF or a Spar, there will always be some element of pitch inaddition to the heave pay-out. This will result in less overall forcecombined from the heave compensators, as they will break out fromsticktion at an earlier point in time. Hence the Xmas tree deck will bethe last object to move when there is a riser pay-out or pay-insituation.

Internal cylinder cushions installed in the heave compensators willrapidly increase the HF stiffness as the cylinders approach the end oftheir stroke range. This will create a rapid rise in force acting on theentire Xmas tree deck 118. The force will overcome the LF sticktion andspring resistance, and drive the deck 118 up or down during extremedisplacement situations. Normally a limited number of heave compensators117 will reach into their hardening zone, as there always will be somepitch differential across the deck. The corresponding risers 116 will besubject to some short duration increase in riser tension which willcause the required additional force to move the deck as mentioned above.

When there are operational requirements to re-position the Xmas treedeck 118 at specific elevations, active control of the deck isnecessary. Various methods to control the cylinders 124 of the deckadjustment system have been examined. One such method is to put ahydraulic pressure precharge in the cylinders 124 when setting up thesystem in the preferred null operating position. By pumping fluid in andout of the cylinders, lowering or raising of the deck 118 can beachieved.

Additional sophistication to the control system might include a PLCprogram that automatically adjusts the elevation of the Xmas tree deck118. Rod stroke measuring devices would be installed on selected heavecompensators 117 and deck support cylinders 124. The PLC would monitorthe stroke position of the heave compensator cylinders, and, when thecylinders reach preset stroke out or stroke in limits, the PLC woulddrive the Xmas tree deck in the required direction by pumping fluid intothe deck support cylinders. In addition, pressure in all cylinders canbe monitored and compared. If rapid pressure drops or cylinder motionsare recorded, (relative to other cylinders), alarms will indicate whereto perform system checks.

The control system philosophy is illustrated in FIG. 13. An Xmas treedeck 118 is supported on four pairs of hydraulic cylinders 124,comprising the LF compensation system. Each pair of cylinders issupplied from a reservoir 127 through a pump 128 and precision controlvalve 129. The inactive (lower) ends of each pair of cylinders arelinked to an accumulator 130 and constitute the passive system thatbalances the self-weight and riser loads on the Xmas tree deck 118.

There is also a marginal overload on the system which is counteracted bythe pressure in the active system in the annulus 131 (above the piston).Margins are set so that there is always a positive over pressure in eachannulus. Hence the passive system acts as a pre-compressed spring.Vertical movement of the deck 118 is effected by the active systemthrough the precision control valves. Any loss or failure of the activesystem leads to a release of the active pressure from the annulus, sothat the system becomes wholly passive. An effect of this arrangement isthat the risers are slightly overtensioned.

To set the system deliberately in passive mode, the active system isdeactivated. The heave compensators 117 of the HF system have increasedspring rates at the ends of their strokes. The Xmas tree deck 118 ismoved by the heave compensators 117 reaching the ends of their strokes.

Typical time histories are shown in FIGS. 14 and 15 (with verticalmotions of the risers exaggerated). FIG. 14 shows a trajectory ofriser/substructure heave and Xmas tree deck displacement with controlledfriction force. The entrapped curve is the relative heave betweensubstructure and riser, while upper and lower curves 133/134 areoverlaid Xmas tree deck movements. The lower overlaid curve shows, forillustrative purposes, the interaction of the heave compensatorstroke-out limitation and the resulting Xmas tree deck downward actingresponse. The upper overlaid curve shows the corresponding up-strokeinteraction. FIG. 15 shows a similar trajectory in a low friction mode.(Reference numerals correspond with those in FIG. 6.) As long as thestroke is within the capacity of the heave compensator, the Xmas treedeck remains static (FIG. 14) or drifts towards nominal equilibrium(FIG. 15). The effect of large substructure pitch or heel angles willlead to Xmas tree deck motion responses at an earlier stage thancompared to an upright position of the substructure. (A larger part ofthe “average” stroke will be taken by the Xmas tree deck adjustmentsystem).

Response of a floating substructure to a seastate may be ‘tuned’ bylocking off one of the systems to harden the heave stiffness. With theXmas tree deck locked in position there is a high spring rate (very hardspring). By releasing all the constraints, the spring rate is lowered togive a very soft spring. Typically, locking all the deck cylinders givesa global stiffness of 875 kN/m, while opening all the cylinders gives aglobal stiffness of 1437 kN/m. In this way the platform eigenvalue inheave may be adjusted by several seconds. This may be particularlyeffective for DDFs and semisubmersibles, for which the waterplanestiffness is small. Increasing the spring stiffness adds to thewaterplane area stiffness. Fine weather would have a stiff system, andrough weather would have a softer system.

ADVANTAGES OF THE INVENTION

The systems described by way of example have several potential benefits.

The two tier riser tensioning system is an appropriate approach forsolving the riser tensioning problem in deep to ultra deep waters. Byapplying the HF/LF philosophy one may design a versatile system withsubstantial user friendliness and safety features, even in the abandonedcase.

The system is very compact and fits easily into a traditionalsubstructure with drilling on top. Work-over and installation operationsmay benefit from the adjustable Xmas tree deck feature. Gaining accessto the individual production trees becomes much easier as the height ofaccess platforms above the Xmas tree deck will be moderate at all times.Inspection of the tensioner rings may be performed in moderate weatherby simply lowering the deck into max down stroke position. By cuttingstroke lengths in half, the two tier system becomes significantly morecost effective than single stroke systems. It may also challenge thecost of buoyancy can systems.

The requirement for long stroke heave compensators is eliminated. Theproposed system contains its own provisions for redundancy, to accountfor partial failures within the system. The proposed system can be selfcontrolled in a stable state, and can be arranged to work on its ownwith simple mechanical control devices.

The proposed system uses proven technology. The heave compensators arealready available for drilling risers used in drill ships operating inwater depths of 3,000 m. A typical weight of 1,400 Tonnes could becompensated on a single stroke wire sheave system.

What is claimed is:
 1. An arrangement to tension a plurality of risersextending from a sea bed up to a substructure for a floating oil/gasdrilling/production platform, the arrangement comprising a conventionalhydraulic tensioner/heave compensator for each riser in which there is asoft spring formed by a piston cylinder combination acting against anaccumulator, the heave compensators for the risers being disposed tocompensate for vertical oscillations of relatively short period betweenthe risers and a Xmas tree deck which supports the heave compensators,wherein there is a vertical position adjustment system having means forintermittently adjusting a vertical position of the Xmas tree deck as awhole relative to the floating substructure, in order to compensate forlonger term changes which would otherwise cause individual ones of theriser's tension or stroke position to depart from a predetermined targetvalue/range; the Xmas tree deck being normally disposed at oneparticular position within its range of movement to compensate for thelonger term changes, such that the heave compensators form a first stageof a two stage heave compensation system, and the vertical positionadjustment system forms the second stage of the two stage heavecompensation system.
 2. An arrangement as claimed in claim 1, in whichthe vertical position adjustment system includes stiff hydraulics whichinterconnect the Xmas tree deck and the substructures.
 3. An arrangementas claimed in claim 2, in which hydraulic oil is supplied frompressurized accumulators when raising the Xmas tree deck and bled to atank when lowering the Xmas tree deck.
 4. An arrangement as claimed inclaim 1, in which the Xmas tree deck has counterbalance means such thatits vertical movements to compensate for longer term changes arecounterbalanced, and only minimal force is required to effect verticalmovement.
 5. An arrangement as claimed in claim 4, in which the Xmastree deck vertical position adjustment system comprises at least threepiston cylinder and accumulator combinations acting between the Xmastree deck and the floating substructure and in which the threecombinations are synchronised to avoid excessive tilt of the Xmas treedeck relative to the substructure.
 6. An arrangement as claimed in claim5, in which the cylinders in the combinations are connected to a singleaccumulator so that the Xmas tree deck is sensibly horizontal, and inwhich there is a rack and pinion mechanism which engages with thesubstructure to maintain parallellity of the moving Xmas tree deck withthe substructure at all times, where rack and pinions engage at leasttwo faces of the deck, at right angles.
 7. An arrangement as claimed inclaim 4, in which the vertical position adjustment system comprises atleast three pulley systems acting between the Xmas tree deck and thesubstructure and in which the pulley systems are powered to compensatefor longer term vertical changes.
 8. An arrangement as claimed in claim7, in which a part of each pulley system is engaged by a further pistoncylinder combination.
 9. An arrangement as claimed in claim 7, in whichthe pulley systems are powered by hydraulic or electric motors forsynchronous movement.
 10. An arrangement as claimed in claim 4, in whichthere is means whereby synchronism can be effected by hydraulic valvelogic while the vertical position adjustment system is moving.
 11. Anarrangement as claimed in claim 4, in which a locking provision on thevertical position adjustment system is arranged to become unlocked whena heave compensator is approaching the end of its stroke.
 12. Anarrangement as claimed in claim 11, in which predetermined high and lowpressures in the heave compensators are arranged to open valves betweenthe piston cylinder combinations and the accumulators in the verticalposition adjustment system.
 13. An arrangement as claimed in claim 1, inwhich the vertical position adjustment system includes mechanicalengagement devices which interconnect the Xmas tree deck and thesubstructure.
 14. An arrangement as claimed in claim 1, in which thereis a control system including means to control the arrangement tooperate without human intervention.
 15. An arrangement as claimed inclaim 14, in which the control system includes means to adjust theelevation of the Xmas tree deck in response to stroke measuring deviceson at least three of the individual heave compensators, whereby, atpreset limits of compensator stroke, the vertical position adjustmentsystem moves the Xmas tree deck in a sense towards the limit reached onthe individual heave compensator.
 16. An arrangement as claimed in claim14, in which the individual heave compensators have increased springrates near both the limits of travel of the individual heavecompensators, whereby the equilibrium of the balanced Xmas tree deckwill be changed such that the Xmas tree deck moves towards theapplicable limit of travel under the action of the vertical positionadjustment system.
 17. An arrangement as claimed in claim 1, in whichthere is means acting on the balanced vertical position adjustmentsystem under mean force equilibrium such that the Xmas tree deck isnormally retained by frictional forces in one particular position withinits range of movement, and is moved intermittently in direct response toone or several of the heave compensators approaching a limit ofoperation.
 18. An arrangement as claimed in claim 17, in which hydrauliccylinders in the vertical position adjustment system arepre-pressurised, so that the system acts as a precompressed spring whichfails to ‘safe’ if the active drive systems lose pressure.
 19. Anarrangement as claimed in claim 18, in which the heave compensators havean increased vertical spring stiffness as they approach the ends oftheir stroke ranges.
 20. An arrangement as claimed in claim 17, in whichthere is adjustment means to change the characteristics of individualheave compensators, so that both the heave compensators for the risersand the vertical position adjustment system for the Xmas tree deckapproach their limits of operation at the same time.
 21. An arrangementas claimed in claim 1, in which the Xmas tree deck has an integral deckcentralisation system.
 22. An arrangement as claimed in claim 21, inwhich the Xmas tree deck is supported on at least four pairs of verticalposition adjustment systems disposed generally symmetrically about thedeck, and the adjustment systems have means to centralise the deckwithin a generally horizontal aperture in the substructure, such thatindividual heave compensators react lateral loads from individual risersinto the Xmas tree deck, and the Xmas tree deck as a whole iscentralised within the horizontal aperture.
 23. An arrangement asclaimed in claim 22, in which vertical rods guide the Xmas tree deckwithin the horizontal aperture.
 24. An arrangement as claimed in claim22, in which projections from the Xmas tree deck engage vertical guiderails surrounding the horizontal aperture.
 25. An arrangement as claimedin claim 22, in which there are pinions on the Xmas tree deck arrangedto engage vertical racks round the horizontal aperture.
 26. Anarrangement as claimed in claim 25, in which resilient means aredisposed to hold the pinions in engagement with the racks.
 27. Anarrangement as claimed in claim 1, in which the vertical positionadjustment system for the Xmas tree deck has a generally central slotoccupied by drillstring riser tensioner, so to facilitatedrilling/workover.
 28. An arrangement as claimed in claim 27, in whichthe Xmas tree deck is used as a foundation for a drilling risertensioner.
 29. An arrangement as claimed in claim 27, in which the Xmastree deck is used as a foundation for a workover drillstring tensioner.30. An arrangement as claimed in claim 1, in which hoses from individualXmas trees on the Xmas tree deck are led through individual downwardlyopening trumpet sleeves dependent from a platform above the Xmas treedeck.
 31. An arrangement as claimed in claim 1, and consisting ofseveral individual bays of deck grid systems placed onboard thesubstructure to reduce the pitch differential across the riser array.32. An arrangement as claimed in claim 1 in which there is provision tolock off the vertical position adjustment system for the Xmas tree deck,whereby to adjust the vertical heave stiffness of the substructure. 33.A substructure for a floating oil or gas production platform, includingan arrangement as claimed in claim
 1. 34. A method of controlling thetension in risers extending from the sea bed up to the hull of asubstructure for a floating oil or gas production platform, using thearrangement as claimed in claim 1.